id summary reporter owner description type status priority milestone component version resolution keywords cc os architecture failure testcase blockedby blocking related differential
7346 Allow the use of `deriving` for GHC generics kosmikus dreixel "Currently, a class that makes use of generic default methods (via the `DefaultSignatures`) extension can be instantiated by providing an empty instance declaration.
I propose to allow the use of `deriving` as well:
1. Standalone deriving should be usable for a class not only for the specific set of classes supported by GHC now, but in addition for any class, if (
* there's at least one generic default given for a method of the class, and
* there are generic or normal default implementations for *all* methods of the class.
There are a number of advantages of this solution over the empty instance declaration: we make it explicit that something generic is going on here; we ensure at compile time that we're not missing an implementation of a method; and we come syntactically closer to built-in derivable classes.
In cases where a conflict arises between current GHC semantics and the proposed semantics (for example, when newtype-deriving is involved, I guess), I propose to stick with current GHC semantics, but I'm open for other suggestions.
2. I'd also like for normal `deriving` to be useful under the same conditions as above. For normal `deriving` GHC has to figure out the class context automatically. I propose that if normal `deriving` is used, GHC uses the same heuristic for figuring out the class context that it uses for `Eq` in the case of `*`-kinded classes, and for `Functor` in the case of `* -> *`-kinded classes. That may not be optimal or even wrong. But in such cases, standalone deriving can still be used." feature request closed normal 7.8.1 Compiler 7.7 duplicate leather@… hvr@… v.dijk.bas@… cgaebel@… pho@… carter.schonwald@… gregmainland@… Unknown/Multiple Unknown/Multiple None/Unknown #5462